This invention relates to the method and apparatus for manufacturing a kind of pressure sensitive copying sheet specially of two side coated pressure sensitive copying sheet.
Usually the so-called "pressure sensitive copying sheet" consists of these three kinds of basic sheets such as top sheet, middle sheet, and bottom sheet, wherein the top sheet is coated on the underside thereof with a composition consisting mainly of pressure-rupturable microcapsules each enclosing a hydrophobic substance containing an electron donating organic chromogenic material (hereinafter referred to as "color former") as dissolved or dispersed therein, the middle sheet is coated on the upperside thereof with another composition consisting mainly of electron accepting acidic reactant material (hereinafter referred to as "acceptor") which will produce a colored image when contact with the color former and also is coated on the lowerside thereof with the composition of color former, and the bottom sheet is coated on the upperside thereof with the composition of acceptor. Thus, any partial pressing on the upperside of top sheet with a pen or a typewriter will break the microscopic capsules positioned on the pressing, resulting in making the color former react with the acceptor so as to develop a colored image only on the part pressed. One top sheet, at least one middle sheet and one bottom sheet are supperposed in that order to form a set of copying sheet in such a manner that the color former coating layer and the acceptor layer are in contact with each other in each ajoining two sheets. Any partial pressing on the upperside of the top sheet of the thus prepared copying system with a pen or a typewriter will break the microcapsules positioned on the pressing, resulting in making the color former react with the acceptor so as to develop a color only on the part pressed.
As the color former is generally very expensive, it is desired to effectively utilize the color former to the utmost extent by making the surface applied with the acceptor as smooth as possible, thereby obtaining sensitive contact between the color former and the acceptor. It is also desirable to make the surface applied with the color former smooth as well as in the surface applied with the acceptor mentioned above and besides deposit the color former as even as possible relatively on the part near the surface of the sheet without allowing the color former to permeate deeply into the sheet.
In order to meet the above requirement it is known to carry out such a process in the manufacture of the middle sheet that firstly the paper sheet is applied with the acceptor on the upperside thereof and then it is passed through a calender so as to make smooth the surface applied with the acceptor and at the same time to reduce porous spaces in the sheet prior to application of the color former on the underside thereof as the next process.
The conventional process in which the application of acceptor, the smoothing of the applied surface, and the application of color former are separately carried out with the use of the respective devices, therefor independently located involves many disadvantages such as intricacy of the process, much labors required, much space required, conveying means required and gauges required etc.
Recently, a successive coating apparatus including a first coater for the acceptor, a first dryer, a calender, a second coater for the color former and a second dryer was proposed, e.g. as disclosed in U.S. Patent Specification No. 3,632,378 granted to Thomas W. Busch and Japanese Laid-Open Publication No. 37,213 of 1973. This successive coating apparatus involves another disadvantages because the calender included in that apparatus is the so-called machine calender which consists solely of a plurality of metal rolls (chilled rolls). The calendering of the sheet applied with the acceptor by passing it through the nip between one metal roll and the other metal roll is effective to increase the density of the sheet by decreasing porous spaces in the sheet but on the other hand it is not so effective to obtain a complete smoothness of the surface of the sheet since the so-called "chilled marks (calender spots)" are produced all over the surface of the sheet. This will result in not only degrading the appearance of the surface but also causing the desensitizer printed on the surface of acceptor's layer to be extremely uneven resulting in showing an uneven and unsatisfactory desensitizing effect. In order to improve such an uneven smoothness of the surface of the sheet, the number of the nip in the machine calender, the pressure applied and/or the moisture content of the sheet may be increased. Such measures would, however, give a severe load on the sheet of a light weight, resulting in bringing about such unfavorable phenomena on the works as the breaking and wrinkling of the sheet and besides making it impossible to operate the apparatus with a high speed. In addition, since the metal rolls will accompany with heating thereof up to a considerably high temperature during the operation, it will cause relatively thermoplastic acceptor compositions such as those of phenol type and aromatic carboxylic acid type etc. cojointly with the thermoplastic binder of synthetic resin latex to adhere to the metal rolls, resulting in these troubles such as the breaking and wrinkling of the sheet and also uneven color developing and bleeding due to some mechanical and chemical deterioration of the acceptor used.
Further each of the metal rolls in the machine calender is usually provided with a crown to absorb any deformations of the metal rolls under pressure. It is however extremely difficult to form the crown in such a manner that it can respond fully to any changes in the pressing condition. Accordingly the calendering conditions vary during operation at a high speed with the result in producing papers having an uneven thickness and wavy edges.
The primary object of the invention is to provide an improved method for the manufacture of a pressure sensitive copying sheet in which a two side coated middle sheet having a uniform acceptor layer and a uniform color former layer at its opposite sides can be continuously manufactured at a high rate and at a high efficiency.
Another object of the invention is to manufacture the two side coated middle sheet having a good appearance and an excellent performance of color development.
A further object of the invention is to provide a method for the manufacture of a pressure sensitive copying sheet in which various kind of acceptor compositions can be utilized.
A still further object of the invention is to provide an improved apparatus carrying out the method mentioned above.
Other objects and advantages of the invention will become apparent from the following description.